Blood glucose homeostasis is controlled by the concerted secretion of pancreatic hormones from the various cell types in the endocrine portion of the pancreas, the islets of Langerhans. The secretion of insulin and glucagon from the pancreatic β and α-cells, respectively, is regulated by nutrients as well as by autocrine, paracrine, and nervous signals. While the molecular mechanisms involved in insulin secretion are relatively well understood, those regulating glucagon secretion are not as clear. For example, researchers are currently exploring how glucagon can be so effectively released subsequent to relatively modest changes in blood glucose concentration prevailing under normal conditions.
Secretion of glucagon from α-cells is increased when the blood glucose concentration decreases (Unger, 1985; Zhou et al., 2004), but the underlying molecular mechanisms that act on the α-cell to induce glucagon release are not known. It has been proposed that high glucose levels suppress glucagon release by acting directly on α-cells (Gopel et al., 2000; Unger, 1985). Mechanistically this has been explained by the selective expression of voltage-sensitive Na+ channels in α-cells that contribute to the generation of neuronal-like action potentials that are inactivated by prolonged cell membrane depolarization (Gopel et al., 2000). According to this model, the inactivation of these Na+ channels suppresses α-cell activity when glucose levels are high.
Other studies, however, have shown that glucose activates α-cells by mechanisms that mirror stimulus-secretion coupling in β-cells, that is, glucose stimulates α-cells and β-cells alike (Olsen et al., 2005; Wendt et al., 2004). Therefore, inhibitory paracrine signals such as insulin, GABA, and Zn2+ being released from the β-cell, rather than changes in the glucose concentration per se, have been suggested to be involved in the direct regulation of glucagon release (Franklin et al., 2005; Gromada et al., 2007; Ishihara et al., 2003; Kisanuki et al., 1995; Ravier and Rutter, 2005; Rorsman et al., 1989).
However, a relief of inhibitory paracrine signals does not fully explain how relatively minor decreases in glucose concentration so effectively promote glucagon release. Glutamate, a major excitatory neurotransmitter in the central nervous system, is of interest because, unlike most other signals, it may stimulate, not inhibit, glucagon secretion in the islet (Bertrand et al., 1993; Hayashi et al., 2003c). Vesicular glutamate transporters that facilitate glutamate uptake into vesicles are expressed by α-cells (Hayashi et al., 2001) and glutamate is secreted together with glucagon (Hayashi et al., 2003c).
Studies in rodent models have shown a large complexity in glutamate signaling in the islet (Moriyama and Hayashi, 2003). Results in the literature are conflicting, and to some extent the role of glutamate remains enigmatic. For instance, it has been reported that glutamate stimulates glucagon secretion via ionotropic glutamate receptors (iGluRs) (Bertrand et al., 1993), that it inhibits glucagon secretion via metabotropic glutamate receptors (mGluRs) (Uehara et al., 2004), and that it activates mGluRs and iGluRs in β-cells to increase insulin secretion (Bertrand et al., 1992; Bertrand et al., 1995; Storto et al., 2006).
The inventors have clarified the role of glutamate and developed assays and methods based on this discovery. Further, the present inventors have discovered that α-cells require positive feedback loops to produce a full glucagon response.
This application claims priority to U.S. provisional application No. 61/064,564, filed Mar. 12, 2008, which is incorporated by reference herein.